X-ray-induced perturbations of mammalian cell growth and proliferation will be studied, with emphasis on DNA synthesis and cell killing. The relation of these two manifestations of radiation insult will be examined in terms of a general model involving potentially lethal damage and its repair. The effects of different concentrations of caffeine on the survival of irradiated cells and on the suppression of radiation-induced inhibition of cell progression will be examined. The relation between the potentially lethal damage expressed by treatment of irradiated cells with two different agents or conditions will be studied.